The present invention is directed to a gaseous fuel burner for process heating. In particular, the present invention is directed to a burner for process heating which yields ultra low nitrogen oxides (NOx) emissions.
Energy intensive industries are facing increased challenges in meeting NOx emissions compliance solely with burner equipment. These burners commonly use natural gas as a fuel due to its clean combustion and low overall emissions. Industrial burner manufacturers have improved burner equipment design to produce ultra low NOx emissions and call them by the generic name of xe2x80x9cLow NOx Burnersxe2x80x9d (LNBs) or various trade names. Table I (Source: North American Air Pollution Control Equipment Market, Frost and Sullivan) gives the LNB market share based on industry for the year 2000. An objective for new burners is to target the industrial sectors that have the largest need for LNBs based on geographic region and local air emission regulations.
As shown in Table I, public utilities and refineries (Chemical and Petroleum Industries) utilize the largest share of low NOx burners. These burners are used in industrial boilers, crude and process heaters (atmospheric and vacuum furnaces) and hydrogen reformers (steam methane reformers).
Nitrogen oxides (NOx) are among the primary air pollutants emitted from combustion processes. NOx emissions have been identified as contributing to the degradation of environment, particularly degradation of air quality, formation of smog (poor visibility) and acid rain. As a result, air quality standards are being imposed by various governmental agencies, which limit the amount of NOx gases that may be emitted into the atmosphere.
Primary goals in combustion processes related to the above are to (1) decrease the NOx emissions levels to  less than 9 parts per million by volume (ppmv) and (2) improve the overall heat transfer uniformity and combustion efficiency of process heaters, boilers and industrial furnaces. For example, in southern California, for process heaters with a firing capacity greater than 20 MM Btu/hr, it is required that the NOx emissions be less than 7 ppmv and that the exhaust gas stream from the process heaters must be vented to a Selective Catalytic Reduction (SCR) unit. At present, this is only possible using best available control technology such as an SCR system. The SCR systems use post treatment of flue gas by reaction of ammonia in the presence of a catalyst to destruct NOx into nitrogen. In addition, California law also requires a fixed temperature window (600xc2x0 F. to 800xc2x0 F.) for  greater than 90% NOx removal efficiency as well as the avoidance of ammonia slip below 5 ppmv. A typical SCR unit for a 100 million Btu/hr process heater would cost approximately $700,000 in capital costs with annual operating costs of $200,000. See, for example, Table 2 of R. K. Agrawal and S.C. Wood, xe2x80x9cCost-Effective NOx Reductionxe2x80x9d, Chemical Engineering, February 2001.
The above compliance costs create a higher cost burden on furnace/process plant operators or utility providers. Generally, emission control costs are transferred to the public in the form of higher overall product costs, local taxes and/or user fees. Thus, power utilities and process plants are looking for more cost effective NOx reduction technologies that would control NOx emissions from the source and do not require post treatment of flue gases after NOx is already formed.
In order to comply cost-effectively for NOx emissions, many combustion equipment manufacturers have developed LNBs. See, e.g., D. Keith Patrick, xe2x80x9cReduction and Control of NOx Emissions from High Temperature Industrial Processesxe2x80x9d, Industrial Heating, March 1998. The cost effectiveness of an LNB compared to the SCR system would generally depend on the type of burner, consistent NOx emissions from burner, burner costs and local compliance levels. In many ozone attainment areas, the LNBs (for  greater than 40 MM Btu/hr) have not been capable of producing low enough NOx emissions to comply with regulations or provide an alternative to SCR units. Therefore, SCR remains today as the only best available control technology for large process heaters and utility boilers.
The greatest challenge in designing a low NOx burner is keeping NOx emissions consistently at sub 9 ppmv level or comparable to NOx emissions at the outlet of the SCR system. The prior art includes low NOx or ultra low NOx burners that produce low NOx emissions using various fuel/oxidant mixing techniques, fuel/oxidant staging techniques, flue gas recirculation, stoichiometry variations, fluid oscillations, gas rebuming and various combustion process modifications. However, most burners are unable to produce NOx emissions at less than 9 ppmv and those that do so in a lab, cannot reproduce such NOx levels in an industrial setting. The technical reasons or challenges in designing a sub 9 ppmv low NOx burner will become evident as described below.
Most large capacity gaseous fuel fired industrial burners used for process heating applications are nozzle mixing type burners. As the name implies, the gaseous fuel and combustion air do not mix until they leave various fuel/oxidant ports of this type of burner. The principal advantages of nozzle mix burners over premix burners are: (1) the flames cannot flash back, (2) a wider range of operating stoichiometry; and (3) a greater flexibility in burner/flame design. However, most nozzle mix air-fuel burners require some kind of flame holder/arrester for maintaining flame stability. One prior art generic nozzle mix burner is shown in FIG. 1, where a metallic flame holder disk is used for providing flame stability. Here, combustion air is induced surrounding the main fuel pipe with flame holder in a large box type burner shell.
The example burner of FIG. 1 also uses staging fuel for secondary combustion to reduce overall NOx formation. However, for successful staged combustion processes, it is very important to have a stable primary flame attached to the flame holder. FIG. 2 shows a typical flame holder geometry in which a multiple-hole fuel nozzle is located in the center and several perforated slots are used on the flame holder conical disk outside for passing through a small amount of combustion air for mixing with the injected fuel. The bluff body shape flame holder creates an air stream reversal as shown in FIG. 2. The opposite direction air stream creates almost stagnant condition (zero axial velocity) for air fuel mixing at the inside cavity of the flame holder cone. This stagnant air-fuel mixture with almost no positive firing axis velocity component is used for attaching the main flame to the flame holder base.
Flame holders of various hole patterns and external shapes (conical, perforated disk, ring, etc.) are used for anchoring flames. For example, U.S. Pat. No. 5,073,105 (Martin, et al.) and U.S. Pat. No. 5,275,552 (Schwartz et al.) describe low NOx burner devices where such flame holders are used to anchor the flame. In U.S. Pat. No. 5,073,105, a primary fuel (30-50% of total fuel) is injected radially inwardly over the flame holder disk with flue gas entrainment (through a hole in the burner tile) for anchoring the primary flame. The remaining, secondary fuel is injected surrounding and impacting the external burner block (tile) surface for fuel staging and furnace gas recirculation. Combustion air mixing with the primary fuel takes place inside the burner block over the flame holder and some NOx is formed due to limited heat dissipation volume inside the burner block cavity and due to creation of locally fuel rich regions.
A very similar approach involving flame holder, primary fuel and secondary fuel injection is used in U.S. Pat. No. 5,275,552. Here, the primary gas, with entrained furnace gas through holes in the burner tile, is swirled in the burner block cavity for better mixing. The swirling primary fuel/flue gas mixture enables better flame anchoring on the flame holder surface.
A main disadvantage associated with flame holders for use in ultra low-NOx burners is localized stagnant zones of fuel-rich combustion that are generally anchored at the inner base of a flame holder cone or disk. These zones are located on the solid ridges between adjacent air slots/holes due to pressure conditions created by the outer air stream. The fuel-rich or sub-stoichiometric mixtures found at the flame holder base for flame stability are unfortunately ideal for formation of Cxe2x95x90N bonds through the reaction CH+N2xe2x95x90HCN+N. Subsequent oxidation of HCN leads to flame holder derived prompt NO formation.
Another main disadvantage associated with flame holders for use in ultra low-NOx burners is limited flame stability if the same burner is operated extremely fuel-lean to avoid prompt NO formation. The overall equivalence ratio (phi) is limited to 0.2 to 0.4 for most flame holder based burners
Finally, a third main disadvantage associated with flame holders for use in ultra-low-NOx burners is that overheating or thermal oxidation of flame holders is quite common due to high temperature flame anchoring, localized reducing atmosphere and scaling on the holder base, and furnace radiation damage when there is an interruption of combustion air supply to the metallic flame holder. In order to overcome the above flame holder disadvantages several attempts have been made in the past. See, for example, U.S. Pat. No. 5,195,884 (Schwartz et al.), U.S. Pat. No. 5,667,376 (Robertson et al.), U.S. Pat. No. 5,957,682 (Kamal et al.) and U.S. Pat. No. 5,413,477 (Moreland). These devices use slight premix combustion or mixing recirculated flue gas (FGR) instead of using a flame holder device (for example, U.S. Pat. No. 6,027,330 (Lifshits)). However, the problems of flash back and limited flame stability range for premix burners (or for FGR burners) do not offer a complete solution in terms of extended stoichiometry, ease of operation, low cost operation and extremely fuel-lean operation (phi  less than 0.1) required for achieving ultra low NOx (e.g.,  less than 5 ppmv) performance. The lack of flame stability is especially detrimental during the startup/heat-up of a process heater/furnace. In a cold furnace, burners with limited flame stability may experience blow-off of flame, thereby creating a hazard and delaying production. A remedy could be to use a second set of burners specially designed for heat-up conditions, which can be costly as well as manpower intensive.
The present invention is directed to an ultra low NOx gaseous fuel burner for process heating applications such as utility boilers, process heaters and industrial furnaces. The novel burner utilizes two unique inter-dependent staged processes for generating a non-luminous, uniform and combustion space filling flame with extremely low ( less than 9 ppmv) NOx emissions. This is accomplished using: (1) a flame stabilizer such as a large scale vortex device upstream to generate a low firing rate, well-mixed, low-temperature and highly fuel-lean (phi 0.05 to 0.3) flame for maintaining the overall flame stability, and (2) multiple uniformly spaced and diverging fuel lances downstream to inject balanced fuel in several turbulent jets inside the furnace space for creating massive internal flue gas recirculation. The resulting flame provides several beneficial characteristics such as no visible radiation, uniform heat transfer, lower flame temperatures, combustion space filling heat release and production of ultra low NOx emissions.
In the present invention, an ultra low NOx burner for process heating is provided which includes a fluid based flame stabilizer which provides a fuel-lean flame at an equivalence ratio in the range of phi=0.05 to phi=0.3 and fuel staging lances surrounding the flame stabilizer with each lance having a pipe having a staging nozzle at a firing end thereof, each lance having at least one hole for staging fuel injection, and each hole having a radial divergence angle and an axial divergence angle. The burner generates NOx emissions of less than 9 ppmv at near stoichiometry conditions.
In one embodiment, the at least one hole and the divergence angles are adapted to provide complete circumferential coverage of the fuel-lean flame. In another embodiment, the at least one hole and the divergence angles are adapted to provide a flat flame pattern. In a third embodiment, the at least one hole and the divergence angles are adapted to provide a load shaping flame pattern
Preferably, between 4 and 16 staging lances are used and each staging nozzle has between 1 hole and 4 holes. Preferably the radial divergence angle is between 80 and 240 and the axial divergence angle is between 4xc2x0 and 16xc2x0. The velocity of fuel exiting the nozzle is preferably between 300 to 900 feet per second for a natural gas staging fuel.
The distance from the forward end of the burner to a point where mixing of staging flame and flame stabilizer flame occurs is preferably approximately 8 to 48 inches. Finally, the fuel rate of the staging for natural gas fuel is from 70% to 95% of the total fuel firing rate of the burner.
The flame stabilizer is preferably a large scale vortex device where the flame has a peak flame temperature of less than approximately 2000xc2x0 Fahrenheit. The equivalence ratio for the flame stabilizer is preferably in the range of phi=0.05 to phi=0.1.
The burner may include a burner block coaxial to the flame stabilizer. Preferably, the burner block is cylindrical or slightly conical, or rectangular in shape.